The invention relates generally to dispersion compensation carried out by phase conjugation and to an optical communication network implemented with phase conjugating means.
In optical transmission systems, an optical signal is modulated with an outbound data stream, and the modulated optical signal is applied to optical fiber. The capacity of the transmission system can be increased in two different ways: by increasing the bandwidth of the data stream or by introducing more wavelengths to be transported in the fiber. The latter alternative is effectively implemented with wavelength division multiplexing (WDM). At the moment, a typical wave range in optical communication is the 1550 nm window. The light to be transmitted is coherent light, that is, the optical signal to be transmitted comprises only a given frequency spectrum having a regular pattern. The most significant properties of the components at the transmitting end of optical fiber include, in addition to the optical power, the breadth of the spectrum generated and the degree of modulation of the signal to be transmitted. The quality and configuration of the components employed is dependent on the purpose of use. Essential components in wavelength division multiplexing include lasers and filters operating at a precise optical wavelength. The window employed determines what kind of laser will be the most suitable for the system to be implemented. The transmitting end of the optical signal has an optical transmitter, usually a laser suitable for that purpose, for generating a coherent photo signal. The receiving end of the fiber has an optical receiver, for example an avalanche photo diode (APD) or a PIN diode which is simpler than an APD.
Various topologies for an optical network can be constructed by concatenating optical links. One such network topology is a network that is physically a ring but logically a mesh (in other words, a connection can be established between any two nodes of the ring). Physically, the optical fiber is connected from node to node (N nodes) to form a ring-shaped structure. There may be more than one optical fiber, and several signals at different wavelengths (xcex1, xcex2 . . . xcexN) may pass in each fiber. Special cases of a ring include a single-fiber ring and a twin-fiber ring. In a single-fiber ring, the signals normally pass in one direction in the fiber, either clockwise or counterclockwise. It is also possible to realize bidirectional traffic in the same fiber. In the case of bidirectional traffic, normally each different wavelength can utilize only one direction in a single link. As a result, in the link between two neighbouring nodes the connection can be set up along the shortest route via a direct link in one direction only, whereas in the connection in the reverse direction the signals must pass along the longer route along the ring. In a twin-fiber ring, the above problem does not appear, since a connection can be established between the pairs of nodes in two different rotating directions. The signals then pass in two fibers in reverse rotating directions, as a rule primarily using the shorter connection. The longer routes exist as redundancy for error situations. For example, if the cable between the nodes concerned is damaged, the connection will not necessarily be cut off, since the longer route can be taken into use.
A commonly used ring network is what is known as the full connection symmetric ring. The ring consists of N network node points and optical fibers between the nodes. In this case, each node has one incoming optical fiber and one outgoing optical fiber. Hence, the ring network is a single-fiber ring in which bidirectional connections are established between all different pairs of nodes in one rotating direction, either clockwise or counterclockwise.
The type of the signal employed in the network can vary; the signal may be e.g. an SDH (Synchronous Digital Hierarchy) or a PDH (Plesiochronous Digital Hierarchy) signal.
The most significant phenomena affecting the signal propagation in optical communication are attenuation and dispersion. With the increase in the number of nodes in the network, also the attenuation increases. When necessary, a linear optical fiber amplifier (OFA), wherewith all different wave lengths passing in the fiber can be amplified simultaneously, can be employed in connection with WDH technology. Optical signals of different frequencies travel at different rates in the fiber, and dispersion is always generated in the fiber. Signal dispersion is one of the factors limiting the size of the ring. There are different implementations for dispersion compensation, perhaps the most widely used being dispersion compensating fibers (DCF) and chirped fiber Bragg gratings. In such a grating, the period of the grating varies linearly as a function of position, the consequence of which is that the grating reflects different wavelengths from different points and thus causes various delays at different frequencies. Dispersion compensating fibers offer negative dispersion in the 1550 nm wave range. The drawback of these fibers is their attenuation-increasing effect. A phase conjugator can also be used for dispersion compensation in optical fiber; the phase conjugator is installed in the system by simply placing it substantially midway the optical fiber with regard to the length of the fiber. Thus, the installation does not require special adjustment or tuning operations. An optical phase conjugator is a device that inverts the spectrum, i.e., performs a frequency shift on the wavelengths arriving at the device by mirroring the incoming wavelength relative to a given mirror wavelength. As a result, each wavelength that has passed through the phase conjugating means has been mirrored in the device in such a way that it is at equal distance from the mirror wavelength as upon arrival at the device, but on the reverse side of the mirror wavelength. The number of phase conjugating means can vary in the network among the different nodes. Thus, for example a receiving node can receive a signal including several different WDM wavelengths, mirrored relative to a given mirror wavelength in such a way that the low and high frequencies are in inverse order compared to the original signal, but on the other hand also a signal that is similar to the original signal from the transmitting node. In such a situation, the receiving node must somehow know how it is to detect incoming signals, since the signals can be either similar to the original signal or mirror images of the original signal. The object of the invention is to remedy the above drawback, so that the receiving node need not know the order of the incoming wavelengths.
The invention relates to dispersion compensation carried out by phase conjugation in an optical communication network. The basic idea of the invention is to implement dispersion compensation in an optical communication network with phase conjugating means in such a way that a route having an even number of phase conjugating means can be found between any two nodes, the result being that the spectrum inversion caused by said means will not present any problems. The set object is achieved in the way disclosed in the independent claim.
A preferred embodiment of the invention is a ring network constructed of two concentric rings in such a way that both rings comprise the same node points, but the number of phase conjugating means to be installed in said rings, in the optical fibers between adjacent nodes is, for example, even in the outer ring and odd in the inner ring. The phase conjugating means can naturally also be installed in the network in a variety of other ways. However, what is essential to the invention is that a signal transmitted from any node in the network can be routed to any other node in such a way that the signal to be transported travels a route in which the total number of phase conjugating means is even, yet so that the signal is primarily routed along a route in which there is the minimum even number of phase conjugating means in said network. However, the solution in accordance with the invention is not limited to a ring network, but the network can naturally have any shape other than a ring, as long as it meets the above condition set on the phase conjugating means.
The solution in accordance with the invention affords two advantages: 1) signal dispersion is compensated for, and 2) the signal is similar at the receiving end and at the transmitting end.